Beginning with the advent of the continuous casting of slabs in the steel industry, the industry has been trying to couple the hot strip mill to the continuous caster through an inline arrangement so as to maximize production capability and minimize the equipment and capital investment required. The initial efforts in this regard consisted of integrating continuous casters producing slabs on the order of 6 inches (15.2 cm) to 10 inches (25.4 cm) with existing continuous or semi-continuous hot strip mills. These existing hot strip mills included a reheat furnace, a roughing train, or a reversing rougher, and a six or seven stand finishing mill with a capacity of 1.5 to 5 million tons per year. This mill arrangement is the present day design of large steel company mills and it is unlikely that new hot strip mills of this design would ever be built due to the high capital cost associated therewith. However, the quest for low cost integrated caster hot strip mills is not solved by current designs. Further, such prior art integrated mills were extremely inflexible as to product mix and thus market requirements.
These difficulties gave rise to the development of the so-called thin slab continuous hot strip mill which typically produces i million tons or less of steel per year as specialized products. These mills have been integrated with thin slab casters which produce slabs on the order of 2 inches (5.1 cm) or less. Such integrated thin slab casters are enjoying increased popularity but are not without serious drawbacks of their own. Significant drawbacks include the quality and quantity limitations associated with the so-called thin slab casters. Specifically, the trumpet type mold necessary to provide the metal for the thin slab can cause high frictional forces and stresses along the surface of the thin slab which leads to poor surface quality in the finished product. Further, the 2 inch (5.1 cm) strip casters are limited to a single tundish life of approximately seven heats because of the limited metal capacity of the mold.
Most importantly, the thin casters by necessity have to cast at high speeds to prevent the metal from freezing in the current ladle arrangements. The required volume of steel also necessitates a relatively high casting speed. Both the trumpet casting nozzle and the relatively high casting speed can create problems in the surface quality of the slab. This relatively high casting speed requires the tunnel furnace which is just downstream of the slab caster to be extremely long, often on the order of 500 feet (152.4 m), to accommodate the speed of the slab and still be able to provide the heat input to a thin slab (2 inches (5.1 cm)) which loses heat at a very high rate. The lengthy furnace can result in increased scale pickup and greater risk of surface flaws in the slab. Since the slab also leaves the furnace at a high speed, one needs the multi-stand continuous hot strip mill to accommodate the rapidly moving strip and roll it to sheet and strip thicknesses. However, such a system is still unbalanced, in terms of capacity of the respective elements, at normal widths since the caster has a capacity of about 800,000 tons per year and the continuous mill has a capacity of 2.4 million tons per year. The capital cost per ton per year then approaches that of the earlier prior art systems that it was intended to replace.
In addition, the scale loss as a percentage of slab thickness is substantial for the 2 inch (5.1 cm) thin cast slab. Because of the extremely large furnace, one must provide a long roller hearth which becomes very maintenance intensive because of the exposed rotating rollers.
The typical multi-stand hot strip mill likewise requires a substantive amount of work in a short time which must be provided for by larger horsepower rolling stands which, in some cases, can exceed the energy capabilities of a given area, particularly in the case of emerging countries. Thin slab casters likewise are limited as to product width because of the inability to use vertical edgers on a 2 inch (5.1 cm) slab. In addition, such casters are currently limited to a single width. Further problems associated with the thin strip casters include the problems associated with keeping the various inclusions formed during steelmaking away from the surface of the thin slab where such inclusions can lead to surface defects if exposed. In addition, existing systems are limited in scale removal because thin slabs lose heat rapidly and are thus adversely affected by the high pressure water normally used to break up the scale.
The conventional casters with inline processing and the known thin slab casters with inline processing fail to provide an adequate combination of broad product mix production levels, quality product and capital expense.
In addition, the prior art thin strip processes can only operate in a continuous manner, which means that a breakdown anywhere in the process stops the entire line often causing scrapping of the entire product then being processed.
It is an object of our invention to integrate a twin slab caster with a continuous processing line. It is a further object to provide a system which balances the rate of the caster to the rate of the rolling mill or mills. It is also an object of our invention to provide a system using less thermal and electrical energy. It is further an object of our invention to provide a system capable of producing up to 2 million tons per year. It is still a further object to provide an automated system with small capital investment, broad product mix, adequate production and quality level, reasonable floor space requirements, reasonably powered rolling equipment and low operating costs.